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Atheism is, in a broad sense, the rejection of belief in the existence God. In a narrower sense, atheism is specifically the position that there is no God. Most inclusively, atheism is the absence of belief that God exist. Atheism is contrasted with theism, which in its most general form is the belief that at least one deity exists.
The term atheism originated from the Greek (atheos), meaning "without god(s)", used as a pejorative term applied to those thought to reject the gods worshipped by the larger society. With the spread of free thought, sceptical inquiry, and subsequent increase in criticism of religion, application of the term narrowed in scope. The first individuals to identify themselves using the word "atheist" lived in the 18th century.

Arguments for atheism range from the philosophical to social and historical approaches. Rationales for not believing in any supernatural deity include the lack of empirical evidence, the problem of evil, the argument from inconsistent revelations, rejection of concepts which cannot be falsified, and the argument from non-belief. Although some atheists have adopted secular philosophies, there is no one ideology or set of behaviours to which all atheists adhere. Many atheists hold that atheism is a more parsimonious worldview than theism, and therefore the burden of proof lies not on the atheist to disprove the existence of God, but on the theist to provide a rationale for theism.

Atheism is accepted within some religious and spiritual belief systems, including Hinduism, Jainism, Buddhism, Raelism, Neopagan movements such as Wicca, and nontheistic religions. Jainism and some forms of Buddhism do not advocate belief in gods, whereas Hinduism holds atheism to be valid, but some schools view the path of an atheist to be difficult to follow in matters of spirituality.

Since conceptions of atheism vary, determining how many atheists exist in the world today is difficult. According to one 2007 estimate, atheists make up about 2.3% of the world's population, while a further 11.9% are nonreligious. According to a 2012 global poll conducted by WIN/GIA, 13% of the participants say they are atheists. According to another study, rates of self-reported atheism are among the highest in Western nations, again to varying degrees: United States (4%), Italy (7%), Spain (11%), Great Britain (17%), Germany (20%), and France (32%).
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